It is known that 5-hydroxy-3-ketoester derivatives are useful as intermediates for preparing blood cholesterol reducing agent [4-hydroxy-3-methylglutaric (HMG) Co-A reductase inhibitor such as trans-6-[2-(2,4-dichlorophenyl)ethyl]-3,4,5,6-tetrahydro-4-hydroxy-2H-pyra n-2-one].
For example, a process for preparing the above blood cholesterol reducing agent using as the 5-hydroxy-3-ketoester derivative (E)-7-(2,4-dichlorophenyl)-5-hydroxy-3-oxo-6-heptenoic acid methyl ester is described in Journal of Medicinal Chemistry [J. Med. Chem., 1985, Vol..28, No. 3, pp. 352].
As a process for preparing the 5-hydroxy-3-ketoester derivative, Chemistry Letters [Chem. Lett., pp. 161, 1975] discloses the following process: Aldehydes are reacted with diketene in the presence of TiCl.sub.4 at -78.degree. C., which then reacted with alcohols, to prepare the 5-hydroxy-3-ketoester derivative. Further, Chemistry Express [Chem. Express, Vol. 6, No. 3, pp. 193-195, 1991] discloses the process that an aldehyde is reacted with diketene in the presence of samarium triiodide at -45.degree. C., which is then reacted with an alcohol at -10.degree. C., to prepare the 5-hydroxy-3-ketoester derivative.
Thus, these processes are required to perform the reaction at extremely low temperatures (-78.degree. C. or -45.degree. C.), and therefore the processes have disadvantages as industrially applicable processes. Further, these processes are employed for preparing a racemic mixture of the 5-hydroxy-3-ketoester derivative, while a process for preparing directly an optically active substance of the 5-hydroxy-3-ketoester derivative from aldehyde and diketene has been unknown so far.